The techniques of charge and discharge control for charging power in storage batteries connected to power systems and discharging power from the storage batteries so that each of the power systems maintains a rated frequency have been conventionally disclosed (see Patent Literature (PTL) 1). PTL 1 discloses the technique of charge and discharge control over a storage battery so that a frequency of a power system is maintained within a predetermined range and a state of charge (hereinafter abbreviated as “SOC”) of the storage battery is also maintained within a predetermined range.